Lucretius in his description of the atomic theory is not generally held to demonstrate a conservation law of matter - the first and most basic conservation law of physics because it relies on the preservation of number; if you have ten stones, then no matter how you rearrange them you still have ten stones.
And this because he says that there are an infinite number of atoms (in Aristotles Physics, an infinite number of principles of first things) and one cannot add up, or sum an infinite number of things.
So one cannot have a global conservation law; still one could arguably have a local one; and this is implicit surely in his description in the following passage from the first book (on matter and void) in On the nature of things (De Rerum Natura) in the translation by AE Stallings:
... Clearly matter is not compressed
Into one heap, because we notice things becoming less,
And we perceive that, over time, everything ebbs and wanes
And old age steals them from our sight, while yet the sum remains
But it is more than an assertion for he appears to provides a rationale, a justification:
This is because the particles that go
From one shrinking object cause another to grow,
Making the former shrivel up, while making the latter flower
Was this actually taken note of in the philosophical literature of Antiquity - or was it just passed over in silence; it's importance and significance not quite understood?
One might argue that the law is implicit in the Parmenidian One: from the One, only One; and it's contrapositive, from Nothing, only Nothing ('for what is not, is not').
But the Parmenidian One isn't matter in our sense; though arguably matter in its aspect of being, is an aspect of the One.